In an oil shear clutch-brake unit of the general type disclosed in U.S. Pat. Nos. 3,638,773 and 3,924,715, which issued to the assignee of the present application, it is sometimes inconvenient to provide a supply of pressurized air from a remote air compressor to actuate the internal piston within the unit. It is also sometimes difficult to obtain a source of clean dry pressurized air for actuating the piston since it is not uncommon for moisture and dirt to collect within the air lines of a manufacturing plant. If the air supply contains moisture, the moisture can produce rust within the clutch-brake unit, especially if no oil is introduced into the air supply and the unit is not operated for an extended period of time. If the air supply contains dirt particles, the particles can prevent smooth operation of the piston and result in excessive wear.
In the art of oil shear clutch and/or brake units with non-rotating actuating pistons, it is known to operate the piston with hydraulic fluid in place of air, especially if a hydraulic pressure line is readily available, for example, from a remote hydraulic pump used to drive hydraulic motors or actuators on the equipment where the clutch and/or brake unit is being used. However, there are many applications and uses for an oil shear clutch and/or brake unit where the supply of clean dry air or hydraulic fluid is not available and would involve significant additional costs to be made available. In such installations, an electrically actuated clutch and/or brake unit may be employed, but such a unit does not offer the important advantages of an oil shear clutch and/or brake unit as disclosed in the above mentioned patents.